The debt is put on Twitter, just like when you buy a house you can put the house itself as insurance for the debt. Elon Musk is still the richest man in the world, your own input to this forum however remains very poor.
I think it's a striking demonstration of the perils of living above your means. If we take the definition of "poor" as the inability to pay for one's monthly upkeep, it's possible for Musk to simultaneously be the richest and the poorest person in the world.
And sure, he could always increase his investment by using more personal funds to make the interest payments, taking the longer term view (whether business or idealistic based). But the fact is that he hasn't, and has been framing this situation exactly as Twitter needing to immediately service that debt.